1. Field of the Invention
The present invention relates to a method of bonding flying leads, and in particular to a method of bonding flying leads to substrate pads using a method of ultrasonic bonding.
2. Background Art
FIG. 3 shows the overall construction of a carriage assembly used in a magnetic disk apparatus. The carriage assembly is formed by attaching suspensions 12, on which magnetic heads are mounted, to the front ends of a plurality of carriage arms 10 provided corresponding to the number of media. An actuator shaft 14 is provided at the base portions of the carriage arms 10 and the carriage arms 10 are rotated parallel to the surfaces of the media with the actuator shaft 14 as a support shaft.
There are a number of methods for electrically connecting the magnetic heads mounted on the suspensions 12 and a signal transfer circuit. FIG. 3 shows a connecting structure that uses so-called “long tail suspension substrates” where the end portions of suspension substrates provided on the suspensions 12 are formed so as to extend as far as attachment positions of a flexible substrate 16 attached to side surfaces of base parts of the carriage arms 10.
With a connecting structure that uses such long tail suspension substrates, substrate pads provided on the flexible substrates 16 and flying leads 18 that extend from base portions of the long tail suspension substrates are aligned and the flying leads 18 are bonded to the substrate pads using an ultrasonic tool. FIG. 4 shows a state where the flying leads 18 and substrate pads 17 provided on the flexible substrate 16 are ultrasonically bonded using a bonding tool 20.
Ultrasonic bonding is used when mounting a semiconductor chip on a substrate using flip-chip bonding or when bonding wires to leads, and a variety of methods have been proposed for reliably carrying out ultrasonic bonding.
Patent Document 1 discloses a method of wire bonding that presses a lead frame via a vibration suppressing member to prevent the lead frame from resonating. Patent Document 2 discloses a method of bonding that applies a conductive material onto substrate electrodes to form bonding electrodes and thereby provide sufficient bonding area. Patent Documents 3 and 4 disclose a method of bonding that interposes an anisotropic conductive film and applies ultrasound in a direction that causes tight attachment. Patent Document 5 discloses a method of bonding that roughens a bonding surface. Patent Document 6 discloses a method that applies a non-conductive bonding agent onto a bonding surface.
Patent Document 1
Japanese Laid-Open Patent Publication No. H10-150137
Patent Document 2
Japanese Laid-Open Patent Publication No. 2005-136399
Patent Document 3
Japanese Laid-Open Patent Publication No. H08-146451
Patent Document 4
Japanese Laid-Open Patent Publication No. H110-189657
Patent Document 5
Japanese Laid-Open Patent Publication No. H05-63038
Patent Document 6
Japanese Laid-Open Patent Publication No. 2005-93581
With the connecting structure that uses long tail suspension substrates shown in FIG. 3, since a large number of flying leads 18 are disposed in parallel at minute intervals, although it is possible to ultrasonically bond the flying leads 18 to the substrate pads 17 one at a time, as shown in FIG. 4, a method that ultrasonically bonds a plurality of the flying leads 18 in a single operation is more efficient.
In FIG. 4, the bonding tool 20 is placed in contact with two flying leads 18 to ultrasonically bond the leads. However, when a method that places the bonding tool 20 in contact with a plurality of flying leads 18 to ultrasonically bond the leads 18 is used, since the contact surface of the bonding tool 20 is formed as a flat surface, if convexes and concaves are present in the bonding surface, the bonding strength will differ at the respective bonding points, resulting in the problem of a lack of reliability for the bonds at the bonded parts.
FIG. 5 schematically shows a state where a bonding interface between the flying leads 18 and the substrate pads 17 is viewed from a cross-sectional direction (i.e., from the side). At the bonding interface, bonding is carried out with convexes in the surfaces of the bonded parts being crushed and an oxide film being damaged, resulting in a state including parts that are actually connected, parts that are connected via an oxide film, and parts A that are not connected.
The flying leads 18 and the substrate pads 17 both have external surfaces that are plated with metal and therefore are connected by bonding metal to metal. Such metal plating layers act so as to absorb any convexes and concaves in the surfaces of the flying leads 18 and the substrate pads 17. However, since the thickness of the metal plating layers is around 3 μm, the metal plating layers are not able to sufficiently absorb the convexes and concaves in the bonding surfaces.